In general, the wheel bearing apparatus to support a vehicle wheel is adapted to rollably support a wheel hub to mount the wheel, via a rolling bearing, and includes driving wheel and driven wheel types. For structural reasons, an inner ring rotation type is used for the driving wheel type. Both the inner ring rotation type and an outer ring rotation type are used for the driven wheel type. A double row angular contact ball bearing has been used in the wheel bearing apparatus since it has a desirable bearing rigidity, durability against misalignment, and a small rotational torque that contributes to improving fuel consumption.
Wheel bearing apparatus is generally classified into first through fourth generation types. In a first generation wheel bearing, it includes a double row angular contact ball bearing fit between a knuckle, forming part of a suspension apparatus, and a wheel hub. In a second generation type, a body mounting flange or a wheel mounting flange is directly formed on an outer circumference of an outer member. In a third generation type, one inner raceway surface is directly formed on an outer circumference of a wheel hub. In a fourth generation type, the inner raceway surfaces are directly formed, respectively, on outer circumferences of a wheel hub and an outer joint member of a constant velocity joint.
FIG. 8 shows one example of a structure of the wheel bearing apparatus. This wheel bearing apparatus is a third generation type used for a driving wheel. It includes an inner member 53 with a wheel hub 51 and an inner ring 52. An outer member 55 is fit onto the inner member 53 via double row balls 54, 54. In the descriptions below, the term “outer side” defines a side that is positioned outside of a vehicle body (left-hand side of FIG. 8) and the term “inner side” defines a side that is positioned inside of a vehicle body (right-hand side of FIG. 8) when the wheel bearing apparatus is mounted on the vehicle body.
The wheel hub 51 is integrally formed with a wheel mounting flange 56 that extend at its outer side end. Hub bolts 56a are secured onto the wheel mounting flange 56 equidistantly along its circumference. The wheel hub 51 is formed with an outer side inner raceway surface 51a on its outer circumference. A cylindrical portion 51b axially extends from the inner raceway surface 51a. The inner ring 52 is formed with an inner side inner raceway surface 52a on its outer circumference and press-fit onto the cylindrical portion 51b. 
The outer member 55 is integrally formed with a body mounting flange 55b on its outer circumference. The body mounting flange 55b is to be mounted on a knuckle (not shown). The outer member inner circumference has double row outer raceway surfaces 55a, 55a. Double row balls 54, 54 are contained between the outer raceway surfaces 55a, 55a of the outer member 55 and the opposing inner raceway surfaces 51a, 52a. The balls 54, 54 are rollably held by cages 57, 57. Seals 58, 59 are mounted on both ends of the outer member 55 to prevent leakage of lubricating grease sealed within the bearing. Also, the seals 58, 59 prevent the entry of rainwater or dusts into the bearing from the outside. The circumference of the body mounting flange is discontinuously formed with three or four projections. Each projection is formed with a screw aperture 69 for a bolt fastened to a knuckle.
A serration 51c is formed on the inner circumference of the wheel hub 51. An outer joint member 61, forming part of a constant velocity universal joint 60, is fit into the serration 51c. The outer joint member 61 has an integral cup-shaped mouth portion 62, a shoulder portion 63, forming a bottom of the mouth portion 62, and a stem portion 64 that axially extends from the shoulder portion 63. A serration 64a, formed on the outer circumference of the stem portion 64, engages the serration 51c of the wheel hub 51. The outer joint member 61 is axially separably connected to the wheel hub 51 by a securing nut 65. The outer joint shoulder portion 63 abuts against the inner ring 52.
An outer circumferential surface 66 of the outer member 55 at the outer side of the body mounting flange 55b is formed with a slight taper and its inner side outer circumference is formed with a cylindrical surface 67 adapted to be fit into the knuckle, as shown in FIG. 9. In addition, a corner portion 68, between the body mounting flange 55b and the outer circumferential surface 66, is formed by forging as a rounded corner with a circular arc cross-section. The inner side fitting surface 67 is formed with a diameter smaller than that of the outer side circumferential surface 66. Thus, bolts fastened to the knuckle can be screwed into the screw apertures 69 from the side of the fitting surface 67. An axially extending recess 70 is milled into the corner 68 at a position corresponding to the screw aperture 69. The screw aperture 69 is formed by tapping after a hole is prepared by drilling.
This remarkably reduces the pitch circle diameter (PCD) of the screw apertures 69. Thus, this reduces the size and weight of the bearing apparatus without diminishing the strength and rigidity of the outer member 55. Also, it enables bolts to be fastened without interfering with the wheel mounting flange 56 of the wheel hub 51 to improve workability during assembly and disassembly of the bearing apparatus. See, Japanese Laid-open Patent Publication No. 91078/2007.
In recent years, there has been a strong desire to improve vehicle fuel consumption by saving material and preventing pollution. In addition to improving fuel consumption, by reducing the size and weight of the bearing apparatus, it is desirable to further reduce the weight of the bearing apparatus to improve the ride comfort by reducing the unsprung mass and to improve the driving stability by improving the road gripping performance of the tires.